The present invention relates to devices and methods for single-sided girth welding. The present invention especially relates to single sided girth welding pipes and tubulars which will be subjected to extreme fatigue conditions.
Single-sided girth welding is used extensively in the fabrication of offshore structures, particularly in risers, which are long sections of pipe. In single side welding, the root, hot, and fill passes are all typically made from the outside of the pipe due to lack of access to or from the inside of the pipe. In addition to lack of access, other factors that cause unacceptable weld profiles include differences in pipe ovality and axial and angular misalignment during fit-up. These geometrically induced difficulties generate unacceptable weld profiles and characteristics which are the main cause of fatigue failures. Accordingly, full penetration welds made from both sides of the pieces to be joined, as opposed to from a single side, are usually recommended in the current art.
In recent years, a number of fatigue failures in single-sided joints (those created with single-sided welding methods) have occurred, mainly caused by fatigue crack initiation at the root of the weld. A root pass in a weld is the first pass performed during the welding operation. Fatigue failure of single-sided welds is a significant problem as cracks initiate and propagate from the root pass on the inner diameter. The principal types of root discontinuities are those due to undercut, incomplete penetration, excess penetration, lack of fusion, or abrupt geometric changes at the junction of the weld and the base material. FIG. 1, a cross-section view of a series of defective welds, demonstrates several types of these defects including undercut defects 1a, incomplete penetration defects 1b, excess penetration defects 1c, and internal concavity or suck back defects 1d. In some installations, backing strips have been used during fabrication of single-sided joints. Generally, backing strips are inserted, bolted, and fitted into the joint interior and welded into place. Backing strips add complexity and cost to welding.
Girth welding is a welding operation where circumferential welds are made between two components. A problem with most girth welded welds is that fatigue failures occur caused in part by an inability to adequately test and inspect the weld by conventional means. Inadequate testing is a problem because conventional testing methods do not reveal the contour or shape of the weld bead in the inner diameter.
Another problem with current single sided girth welding methods is that current art lineup clamps can cause damage to the interior of the pieces being welded, in part because the lineup clamp does not adequately shield against abrasion during insertion and removal of the lineup clamp from the interior of the pieces to be welded. Accordingly, bare metal or other abrasive materials on the lineup clamp can scratch or otherwise damage the interior of the pieces when the lineup clamp is inserted or removed.
Because the potential for root defects in single-sided welds is high, reliable and appropriate non-destructive testing (NDE) procedures are of particular concern. Root defect detection is normally difficult because the normal inspection methods are available only from the outside of the weld area. Most NDE testing includes magnetic particle inspection testing (MT), ultrasonic testing (UT), and/or radiographic testing (RT), but desirable visual inspection of the intrusion of the weld into the interior is difficult.
When riser joints are used underwater, as in deep sea or other offshore wells, wave motions and water currents subject the riser joints to oscillations and cyclic forces, leading to fatigue failure with the cracks typically initiating from the toe of the weld on the inner diameter.
It is therefore an object of the present invention to provide an improved welding apparatus and method, capable of creating and testing single-sided girth welds that are highly fatigue resistant. Accordingly, an apparatus and method for achieving and testing single-sided girth welds which adapt to and compensate for inherent geometrically induced anomalies, such as axial misalignment, angular misalignment, and ovality, which limits defects introduced during the welding process and thus limits fatigue failures is described.